The invention relates to a valve structure for launching and/or retrieving scraper bodies commonly termed "pigs" which travel through a pipeline to dislodge wax and other accumulated deposits from the interior of the pipeline. The particular embodiment of the invention disclosed herein is intended to handle pig balls which may range from two inches to sixteen inches in diameter. The pig ball valve structure may be employed to launch and retrieve pigs of other shapes and other ranges of size.
The many different valve structures that have been devised for this general purpose have various disadvantages and shortcomings which may be understood from the following discussion of the objects of the present invention.